Analysis of Photon-Induced Drain Current in Polycrystalline-Silicon Thin-Film Transistors

Abstract

The electrical properties of illuminated polycrystalline-silicon thin-film transistors (poly-Si TFTs) were investigated by measurement and device simulation. Several features were revealed in n-channel poly-Si TFTs: the flat-band voltage (Vfb) shifts to a positive direction on the order of 0.1 V. The trap density increases around the midgap. In the on-state, the grain boundary barrier height slightly decreases owing to the increase in donor-type and the decrease in acceptor-type trap densities. In the off-state, the reverse current of pn-junction at the drain edge becomes much larger than the generation current or gate-induced drain leakage current, showing almost linear dependence on light intensity. A hump appears in off-current below Vfb, which is derived from the transition of carrier transport from the reverse diffusion at pn-junction in the off-state to the drift-diffusion in the on-state. Considering all these features, a surface-potential-based drain current model was proposed. Calculations using the model were compared with device simulation and measurement by varying the light intensity or drain voltage. The good agreements between them confirm the validity of the model.

A temperature study of photosensitivity in SLS polycrystalline silicon TFTs

“…The results suggest that in the on regime, thus under strong inversion condition, no influence on the barrier height is obtained, an experimental result that supports the simulation results presented in [13].…”

“…The dark current is thermally activated [15]. On the other hand the room temperature results reveal that the photo-leakage current increases proportional to illumination flux [13]. According to this the current under illumination is expected to be expressed as the sum of the current arise by the thermally activated carriers generated also under dark through gap states and the photo-current arise by the photo-generated carriers in the reserved bias p-n junction that formed in this case between the body and the drain, thus…”

“…Across x-direction the scattering on potential barriers and defects is minimised and thus n, is not affected by illumination. Across y-direction the potential barriers heights are affected by the photo-generated carriers [13] leading to different transport properties and thus different value for n. Therefore the polycrystalline material micro-structure must be taken into account if the corresponding application is based on the subthreshold current.…”

“…However the interest on photoconductivity, in particular for poly-Si TFTs, has been revived during last years as indicated by the non-negligible number of published papers by different research groups. These works have been initially focused on presenting devices fabricated by novel fabrication procedures that minimised the undesirable light impact on the below threshold operation [8,9] and more recently on monitoring and modelling the role of light intensity in OFF, subthreshold and ON drain current at room temperature [10][11][12][13][14] aiming to clarify the illuminated device physics.…”